While shooting the .303 in a Krag is probably not wise... the likelihood of shearing off the locking lug and sending the bolt into the shooter's face is doubtful (I refer to the late Michael Petrov's experiments).  Now cracking the lug, receiver or both is more likely, I suspect it would hold up longer than people think.  Again, refering to Petrov... he used a case full of Bullseye before he finally destroyed his Krag, and this was after shooting several loads that far exceeded the recommended max 40,000 CUP.